1. Field of the Invention
The present invention generally relates to an ultrasonic reference or calibration block applicable to a broad range of different materials and material thicknesses as well as to a method for calibrating an ultrasonic inspection system. More particularly, the present invention relates to a such a calibration block and method which reliably and accurately calibrates, for standardization, the signal output of an ultrasonic system.
2. Background Discussion
Digital ultrasonic data acquisition systems are widely used in the non-destructive evaluation (NDE) of materials using pulse-echo ultrasonics. Flaws detectable and charactereristical using pulse-echo ultrasonics include, but are not limited to, material voids, porosity, cracks and delaminations.
As best seen in FIG. 1, such an inspection system, generally indicated at 11, employs a piezoelectric transducer 13 which functions as a transmitter/receiver is stimulated by electrical signals from a pulser circuit 15 so as to generate pulses 17 of ultrasonic sound energy which are transmitted through a coupling fluid (water) toward a part 19 to be evaluated. To obtain the best results, it is preferred that the angle of incidence of the ultrasonic pulses 17 be normal to the surface of the part 19 and the part 19 be submerged in water 20.
As a result of this insonification of the part, some of the incident ultrasonic energy is reflected from the proximal or front surface of the part and some of the pulse is reflected from the distal or back surface of the part in the form of sonic echo returns. However, not all the ultrasonic energy is directly reflected from the proximal and distal surfaces. In this regard, some energy is trapped in the material itself and reverberates back and forth between the front and back surfaces of the part, portions of this trapped energy being emitted on a non-random basis from either the front or back surface. Further, some of the energy is transmitted entirely through the part.
The piezoelectric transducer 13 also detects the sonic echo signals 21 received back from the insonified part 19. These echo signals 21 are converted into electrical signals by the piezoelectric transducer 13, amplified by a receiver or input amplifier 23 and provided to a receiver 25. The output of the receiver 25 is preferably converted into digital form by an analog-to-digital converter 27 for analysis by a computer 29 to non-destructively evaluate the material of the part. The computer is typically provided with a display 31 for at least viewing the signals generated by the receiver 25 and also for plotting the results of the processing.
Because the ultrasonic pulses are reflected from interfaces at different depths, i.e., the front and back surface interfaces of the part, the echo signals from each of the interfaces are spaced in time as to their arrival at the piezoelectric transducer 13. Therefore, each of these different echo returns can be distinguished from one another and viewed as well as plotted on the display 31, the type of material insonified determining the configuration of the echo returns. FIG. 2 illustrates the returns from a piece of acrylic, waveform A representing the reflection from the proximal or front surface of the part and waveform B representing the reflection from the distal or back surface. Typically, it is waveform B which is used to analyze the material of the part 19 because the reflection responsible for waveform B has passed entirely through the part and thus, has encountered all deflects.
The receiver output signals must have some minimum threshold amplitude in order for the computer 29 to be able to use the receiver output signals to detect material flaws in the part 19 because the receiver 25 is usable over only a defined dynamic range and the analog-to-digital 27 converter has a fixed resolution, typically 8 to 10 bits. Therefore, gain G is used as an offset to bring a desired signal within the usable dynamic range of the analog-to-digital converter 21 of the receiver 25. As a result, the gain G of the receiver input amplifier 23 must be set or calibrated in order to provide the required output level of the receiver 25.
To conduct a non-destructive evaluation of a part using an ultrasonic inspection system, the output of the ultrasonic system is first calibrated. The calibration is achieved by insonifying a flawless piece of the material from which the part is made, i.e., a standard.
In calibrating the system, the amplitude of the output signal of the receiver 25 is adjusted by varying the gain G of the receiver amplifier 23 until the displayed signal, i.e., the receiver signal, reaches some arbitrary value, for example, 90% of full scale. 90% full scale is typically chosen as the arbitrary value to provide the desired degree of sensitivity in order to obtain useful results during analysis by the computer.
The adjusted output of the input amplifier 23 is then fixed by setting its gain G at the level that results in the 90% of full scale reading on the display and this output amplitude is used as the standard amplitude level for evaluating all other portions of the part. The frequency and amplitude of the ultrasonic energy received back from each portion of the part is then analyzed, in comparison with the frequency and amplitude response of the sonic energy received from the flawless standard made of the material of the part, to determine the presence or absence of flaws. In this regard, an overall lowering in amplitude of the output of the inspection system, i.e., output of the receiver 25, typically indicates the presence of some kind of material defect.
Because certain material flaws are visible only within specific frequency ranges, it is often necessary to vary the frequency of output pulse of the ultrasonic transducer to obtain the best results. Therefore, it is necessary to have accurate information on the frequency spectrum of the output pulse of the transducer.
Further, different materials have different ultrasonic properties, and, as a result, have different brightnesses, i.e., varying degrees of transmission, attenuation and reflectivity, when insonified with an ultrasonic pulse. Therefore, in order to obtain useful results, it is often necessary to vary the output amplitude of the receiver signal by varying the gain of the receiver amplifier in dependence on the type material and thickness of the material.
Because the output of the inspection system can change from day to day due to drift, component aging, or the like, a given gain setting of the receiver amplifier on one day may not result in the same output amplitude level of the receiver signals on another day. As a result, the output of the inspection system must be standardized on at least a daily basis, sometimes hourly. Additionally, the amplitude level for a given gain setting of the receiver amplifier can vary from inspection system to inspection system which results in the need to be able to accurately and reliably calibrate the output level of any ultrasonic inspection system to a precise standard for comparison of results from different ultrasonic inspection systems.
When the material of the part being evaluated has well-known ultrasonic properties, for example, metal or plastic materials, which are isotropic, have uniform ultrasonic properties which do not change for a given orientation of the lattice structure relative to the ultrasonic transducer, it is fairly easy to calibrate the output amplitude of the system using any piece of the well-known material as a standard. As a result, different tests run on different inspection systems can be compared with one another with a high degree of reliability when the output level of each inspection system is calibrated to the same standard using different pieces of the same material. However, problems are often encountered when the ultrasonic properties of the material of the part are unknown. This problem is typically encountered when using NDE techniques to analyze new and different composite materials which have a wide variation in ultrasonic properties and are not abundantly available. In order to properly calibrate the ultrasonic inspection system when analyzing a part made of a material having unknown ultrasonic properties, a portion of the part under inspection, which is believed to be flawless, i.e., the best area, is insonified and used as the standard for calibration of the ultrasonic inspection system. Thereafter, the portion of the part believed to be flawless is archived as the standard for further reference. This approach is somewhat cumbersome in practice, requiring the storage and indexing of a large number of material samples which may or may not be readily available in quantity.
Alternatively, a calibration block is used as the standard of calibrating the output of the inspection system. An example of a known calibration block arrangement is schematically indicated in FIG. 3. The arrangement comprises an ultrasonic piezoelectric transducer 13 which is scanned in the direction of arrow 33 across a step wedge calibration block 35. Because of the step wedge configuration of the block 33, the ultrasonic echo returns 37 received back from the distal surface 39 of the block 33 travel through increasing quantum amounts of material resulting echo returns of decreasing brightness.
However, drawbacks are encountered by such a scanning arrangement which requires that the position of the piezo-electric transducer 13 be maintained at a constant distance from the distal surface 39 of the block 33 during the entire scanning process in order to obtain useful result. Further, the scanning process introduces unwanted errors and complexities during the calibration process as opposed to a calibration method which requires the insonification of a calibration block at but a single point. Moreover, any variation in the distance between the transducer 13 and the distal surface 39 during scanning results in non-standard echo returns from the distal surface 39. As a consequence, this approach results in a calibration having a limited dynamic range and ultrasonic beam characteristics which may not compare to the actual part being inspected.
Therefore, the need has arisen for an ultrasonic reference or calibration block standard which can be used to calibrate the output of an ultrasonic inspection system to a given standard output, which standard can then be used with materials having unknown ultrasonic properties.